Inhibition of silica and silicate deposition in cooling water systems

ABSTRACT

A method inhibiting the deposition of silica and silicate compounds on the metallic surfaces of cooling water equipment comprising adding to the water a composition consisting of hydroxyphosphonoacetic acid and a polymer having the formula: ##STR1## wherein M is a water soluble cation. This composition may further comprise a molybdate compound.

This is a divisional of application Ser. No. 07/729,519 filed Jul. 12,1991, now U.S. Pat. No. 5,158,685.

FIELD OF THE INVENTION

The present invention relates to cooling and boiler water systems. Thecontrol of silica and silicate deposition within these systems is thefocus of the invention disclosed hereinafter.

BACKGROUND OF THE INVENTION

The problems of scale formation and its attendant effects have troubledwater systems for years. For instance, scale tends to accumulate oninternal walls of various water systems, such as boiler and coolingsystems, thereby reducing heat transfer properties and fluid flowthrough heat exchange tubes.

One particular type of deposit, silica, is especially troublesome insome systems. Where the water used in cooling systems and water-cooledindustrial heat exchangers is taken directly from lakes, rivers, pondsor municipal water sources, various amounts of dissolved and suspendedsolids including silica are present. Problems are compounded in openrecirculating water systems due to the fact that as water evaporates thesilica concentration increases, thereby increasing both the occurrenceand degree of deposition.

In cooling water systems, silica and silicate compounds form deposits onthe internal metal surfaces in contact with the water flowing throughthe system. In this manner, heat transfer efficiency becomes severelyimpeded, which in turn has a deleterious effect on the overall operatingefficiency of the cooling water system. Silica and silicate depositionalso causes problems on other conduit and pipe surfaces as well as onequipment such as valves, nozzles and pumps.

Although current industrial cooling systems make use of sophisticatedexternal treatments of the feedwater, e.g., coagulation, filtration,softening of water prior to its being fed into the water system, theseoperations are only moderately effective. In all cases, externaltreatment does not in itself provide adequate treatment since muds,sludge, silts and dissolved solids such as silica, escape the treatment,and eventually are introduced into the cooling system.

Various methods have been utilized for resolving the problem of sludgeand silt, including silica, deposition. In U.S. Pat. No. 3,578,589, Hwaet al., inhibition of scale, mud, silt and sludge deposition is achievedby adding a nonionic surface active agent, such as a polyethyleneoxyalkyl phenol, and a water soluble polymer, such as polyacrylic acid.

In Watsen et al., U.S. Pat. No. 3,948,792, the patentees disclose theproblem of silicate scale formation in automobile and diesel coolantsystems. They teach adding a water soluble carboxylic acid polymer andnitrites along with either boric acid or borates.

U.S. Pat. No. 4,869,845, Chen, utilizes the same copolymer as utilizedin the present invention to treat scale and corrosion problems incooling and boiler water systems. The copolymer is added to the systemwith both a phosphonate and a zinc compound. The purpose of thecopolymer is to maintain the solubility of zinc. Without this mechanism,the zinc would precipitate in the form of zinc hydroxide and would beunavailable for its desired anti-corrosion activity.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, it has been discovered that atreatment program comprising water soluble copolymers as shown inFormula I hereinafter and hydroxyphosphonoacetic acid is effective incontrolling the formation of silica and silicate deposits on theinternal surfaces of structures housing cooling water systems. ##STR2##M is a water soluble cation. This polymer is referred to as acrylicacid/allyl hydroxy propyl sulfonate ether (AA/AHPSE). The IUPACnomenclature for AHPSE is 1-propane sulfonic acid 2-hydroxy-3-(2propenyl-oxy) mono sodium salt.

The number average molecular weight of the water soluble copolymers ofFORMULA I may fall within the range of 1,000 to 1,000,000. Preferablythe number average molecular weight will be within the range of fromabout 1,500 to about 10,000. The key criterion is that the polymer bewater soluble.

The molar ratio of repeat units a:b in FORMULA I may fall within therange of between about 30:1 to 1:20, with the a:b molar ratio range offrom about 10:1 to 1:5 being preferred.

With respect to both repeat units of the polymers of the presentinvention, they may exist in acid or water soluble salt form when usedin the desired water system.

As to preparation of the monomer designated as a above, in FORMULA I,acrylic acid is well known. It may be produced by hydrolysis ofacrylonitrile or via oxidation of acrolein.

Turning to the allyl containing monomer, monomer b, in FORMULA I above,it may be produced by reacting allyl alcohol with a non-tertiary alcoholin the temperature range of about 25°-150° C. as detailed in U.S. Pat.No. 2,847,477 (the entire disclosure of which is hereby incorporated byreference) followed by, if desired, sulfonation, phosphorylation,phosphonation or carboxylation of the monomer via well-known techniques.

The preferred allyl hydroxyl propyl sulfonate ether monomers (monomer b,FORMULA I) may conveniently be prepared via a ring opening reaction ofthe epoxy group of an allyl glycidyl ether precursor. Sulfonation of theepoxy group with sodium sulfite in the presence of a phase transfercatalyst such as tetra-n-butyl ammonium bisulfite or with fumingsulfuric acid containing sulfur trioxide will produce the sulfonic acidgroup and hydroxy group of the AHPSE. The resulting monomer can befurther neutralized with caustic or other basic material. The reactionis illustrated by the following mechanism: ##STR3##

It should be noted that monomer b may itself be allyl glycidyl etherwhich is available from several commercial sources. Suitable cations, M,include Na+, Ca+² and K+.

After the desired monomers have been obtained, free radical chainaddition polymerization may proceed in accordance with conventionalsolution polymerization techniques. Polymerization initiators such aspersulfate initiators, peroxide initiators, etc. may be used. Preferablythe requisite monomers are mixed with water and alcohol (preferablyisopropanol). The resulting polymer may be isolated by well-knownmethods such as distillation, etc., or the polymer may simply be used inits aqueous solution.

The treatment program of the present invention comprises adding thecopolymer (AA/AHPSE) previously described along withhydroxyphosphonoacetic acid (Belcor 575 available from Ciba-Geigy) tothe aqueous system to be treated. These compounds are added in aneffective amount for the purpose of inhibiting silica and silicatedeposition. The amount of AA/AHPSE added should be sufficient tomaintain a concentration of from 10 to 100 ppm (active). Thehydroxyphosphonoacetic acid is added in an amount sufficient to maintaina concentration of from 3 to 20 ppm (active).

The relative component concentrations will vary depending upon theparticular aqueous system to which the treatment is directed. Factorsinfluencing dosages are the surface area of the heat exchange surfaces,pH, temperature, water flow rate and concentrations of potential depositforming species.

With such wide variances in the chemistry of makeup water, some systemsare more difficult to treat than others. It has been discovered that theaddition of a molybdate compound to the aforementioned treatment programprovides the necessary efficacy, to inhibit silica and silicatedeposition in these systems. The molybdenum may be added in any suitableform, such as its oxides. Preferably, MoO₄ ²⁻ is utilized and would beadded to the aqueous systems in a sufficient amount so as to maintain aconcentration of from about 10 to 50 ppm as MoO₄.

The treatment of the present invention may be added to the aqueoussystem to be treated either on a continuous basis or intermittently. Theseparate compounds comprising the treatment program may be preblendedprior to addition to the aqueous system or each compound may be addedseparately according to a predetermined dosage for the purpose ofachieving the desired concentration level of the treatment compounds inthe aqueous system. When preblended, the composition would be comprisedof the following hydroxyphosphonoacetic acid: polymer: Molybdate (asMoO₄) weight ratio--6:30:20

EXAMPLES

Recirculator tests are used to simulate heat transfer conditions incooling systems. The recirculator test units have been used todemonstrate the inhibition of silica and silicate deposits. These unitshave a volume of approximately 11 liters and utilize a pump to generatewater flow past the outside of a metal tube that contains a heater. Theunits have a temperature control device to maintain a desired sumptemperature. The pH is maintained by the controlled addition of CO₂ tothe system. The units are capable of being operated in either anevaporative or non-evaporative mode.

In the evaporative mode, the sump is charged with a specific water and adilute water is fed to maintain the specific water composition. Thedilute makeup maintains the system volume and compensates forevaporation and selected blowdown rates. The system volume is maintainedby a level controller and a blowdown pump.

In the non-evaporative mode, two makeup solutions are fed simultaneouslyto the units to maintain the specified water composition. The systemvolume is controlled by an overflow port. One makeup solution containscalcium chloride and magnesium sulfate and the other makeup solutioncontains sodium bicarbonate and sodium silicate.

In the evaporative mode, silica deposition was evaluated using waterhaving the following composition:

    ______________________________________                                                  1820 ppm Ca as CaCO.sub.3                                                      840 ppm Mg as CaCO.sub.3                                                     3670 ppm SO.sub.4                                                             1290 ppm Cl                                                                   1597 ppm Na                                                                    250 ppm SiO.sub.2                                                             500 ppm M alkalinity                                               ______________________________________                                    

The physical characteristics of the recirculator test units used in thisevaluation were as follows:

pH 8.2

120° F. sump temperature

308 watts on the heater (@ 13,000 BTU/hr/ft² heat flux)

2.8 ft/sec water velocity past the heated tubes mild steel coupons andheater tubes

Table I shows the results of an evaluation of the treatment programaccording to the present invention under the above noted testconditions. The value of hydroxyphosphonoacetic acid is apparent since acomparative test was conducted using, in its place,1-hydroxyethylidene-1,1-diphosphonic acid (HEDP, Dequest 2010).Tolyltriazole was added in these tests because tolyltriazole istypically used to prevent copper and brass corrosion in cooling systems.

                  TABLE I                                                         ______________________________________                                        Evaporative Recirculator Test                                                               Concen-                                                                       tration                                                                              Results                                                  Composition     (active) Tube      Coupons                                    ______________________________________                                        hydroxyphosphonoacetic acid                                                                    6 ppm   no        no                                                                  deposition                                                                              deposition                                 polymer *       30 ppm                                                        tolyltriazole    3 ppm                                                        HEDP             6 ppm   deposition                                                                              deposition                                 polymer *       30 ppm                                                        tolyltriazole    3 ppm                                                        ______________________________________                                         * polymer: acrylic acid/AHPSE, 3/1 mole ratio, Mw @ 8,000.               

The tube and coupon for the HEDP composition test were analyzed byFourier Transform Infrared Analysis (Diffuse Reflectance.) The couponwas also analyzed by Scanning Auger Microscopy (SAM). The infraredanalysis showed a large peak at 1067 cm⁻¹ with small peaks at 1636, 1558and 1420 cm⁻¹ for the coupon deposit. The infrared analysis showed alarge peak at 1084 cm⁻¹ with minor peaks at 1650 cm³¹ 1 and 799 cm⁻¹ forthe tube deposit. The SAM analysis showed the following atomicconcentrations: 38.43%C, 32.41% 0, 11.21% Si, 8.42% Ca, 4.78% Mg, 2.89%Fe, 1.13% Na, 0.52% S and 0.21% Cl. No phosphorus was detected. Anotherlocation on the coupon was detected to contain 23% Si. The infrared andSAM analysis showed that the deposit was a silicate with some silica.Since silica and silicate are supersaturated in the test water it wouldbe expected that they would deposit on heat transfer surfaces andcoupons under the above test conditions. Such was the case with the HEDPtreatment sample. However, the sample with the hydroxyphosphonoaceticacid exhibited no deposition in contrast to what would have beenexpected.

Studies with different test waters show that the ability ofhydroxyphosphonoacetic acid and polymer to inhibit silica and silicatedeposits can be improved by the addition of molybdate. Innon-evaporative recirculator units, tests were conducted with thefollowing water conditions:

    ______________________________________                                                  600 ppm Ca as CaCO.sub.3                                                      200 ppm Mg as CaCO.sub.3                                                      426 ppm Cl                                                                    192 ppm SO.sub.4                                                              243 ppm Na                                                                    500 ppm M alkalinity                                                          100 ppm SiO.sub.2.                                                  ______________________________________                                    

Physical characteristics of the test units were as follows:

pH 9.0

120° F. sump temperature

308 watts on the heater (@ 13,000 BTU/hr/ft² heat flux)

mild steel heater tubes and,coupons

Table II shows the results of testing under the above conditionscomparing a silica deposition treatment without molybdenum to one withmolybdenum.

                  TABLE II                                                        ______________________________________                                        Silica Deposition Inhibition w/Molybdate                                                      Concen-                                                       Treatment       tration  Results                                              Composition     (active) Tube      Coupons                                    ______________________________________                                        hydroxyphosphonoacetic acid                                                                   12 ppm   deposit   deposit                                    polymer *       50 ppm                                                        hydroxyphosphonoacetic acid                                                                   12 ppm   no        slight                                                              deposit   deposit                                    polymer *       50 ppm             on                                         MoO.sub.4       20 ppm             edges                                      ______________________________________                                         * polymer: AA/AHPSE, 6/1 mole ratio, Mw @ 8,000.                         

In order to verify the composition of the deposits, the first of theabove tests was repeated under the same conditions except that silicawas absent from the water. In this case, no deposition developed on thetubes or coupons.

While this invention has been described with respect to particularembodiments thereof, it is apparent that numerous other forms andmodifications of this invention will be obvious to those skilled in theart. The appended claims and this invention generally should beconstrued to cover all such obvious forms and modifications which arewithin the true spirit and scope of the present invention.

I claim:
 1. A composition for inhibiting the deposition of silica and silicate compounds in a cooling water system comprising hydroxyphosphonoacetic acid, molybdate and a polymer having the formula ##STR4## wherein M is a water soluble cation and the molar ratio of a:b is from about 30:1 to 1:20.
 2. The composition according to claim 1 wherein the weight ratio of hydroxyphosphonoacetic acid: polymer: molybdate is 6:30:20.
 3. The composition according to claim 1 wherein the molybdate is MoO₄ ²⁻.
 4. The composition according to claim 1 wherein M is selected from the group consisting of Na⁺, NH₄ ⁺, Ca⁺² and K⁺.
 5. The composition according to claim 1 wherein the molar ration of a:b is from about 10:1 to 1:5. 